1. Field of the Invention
The disclosure relates in general to a memory circuit. In particular, the disclosure relates to magnetic random access memory (MRAM) circuits.
2. Discussion of the Related Art
Magnetic Random Access Memory (MRAM) is a non-volatile memory used in long term data storage. The magnetic memory cell of MRAMs stores information by its magnetoresistance. When data is to be written to a magnetic memory cell, the magnetic memory cell is selected by providing magnetic fields respectively produced by a selected word line and bit line, and combined magnetic fields change the magnetic vector of the magnetic memory cell to change the resistance of the selected magnetic memory cell. When data is to be read from the selected magnetic memory cell, a sensing current passes through the selected magnetic memory cell, and the stored data is obtained according to the voltage difference across the selected magnetic memory cell.
FIG. 1 is a schematic diagram of a conventional MRAM array. The conventional MRAM array comprises bit lines B1˜B3 and word lines W1˜W3. The magnetic memory cell located at the intersection of one bit line and word line comprises a soft magnetic layer (free ferromagnetic layer), a tunnel barrier layer, a hard magnetic layer (pinned ferromagnetic layer) and a non-magnetic conductor. The relative magnetic orientations of the free ferromagnetic layer and the pinned ferromagnetic layer represent logic values of “0” and “1”, for example.
The magnetic memory cell is written to by the magnetic field generated by the selected word line and bit line. Thus, only the magnetic dipole moment of the selected magnetic memory cell is switched. For example, the magnetic fields generated by bit line B1 and word line W1 switch only the magnetic dipole moment of magnetic memory cell Cs, without changing the state of non-selected magnetic memory cells C12, C13, C21, and C31.
FIG. 2 shows a relationship between switching state of the magnetic memory cell and magnetic fields provided by the word line and bit line, referred to as an “asteroid curve”. Horizontal magnetic field Hl is provided by the bit line, and vertical magnetic field Ht by the word line. According to the asteroid curve, the resistance of magnetic memory cell is switched when the horizontal magnetic field Hl is H0 without vertical magnetic field Ht. If vertical magnetic field Ht exists, a lower horizontal magnetic field Hl, less than H0, is able to switch the resistance of the selected magnetic memory cell.
In FIG. 2, the resistance of the selected magnetic memory cell is not switchable in area A. The selected magnetic memory cell is switched when outside the area A, to a first state (high resistance as an example) when the applied magnetic field is in the first quadrant, and to a second state (low resistance as an example) when in the second quadrant.
In an MRAM array, a reference magnetic memory cell is arranged for a predetermined number of bit lines (32 or 64). To read the selected magnetic memory cell Cs, a read current through data line D1, magnetic memory cell Cs and bit line B1 is provided. Another reference current through reference data line Dr, reference magnetic memory cell Cr and reference bit line Br is also provided. Data stored in magnetic memory cell Cs is obtained by comparing the voltage level at bit lines B1 and Br.
The asteroid curve of the conventional reference magnetic memory cell Cr is identical to the selected magnetic memory cell Cs, that is, the switch conditions of conventional reference magnetic memory cell Cr and magnetic memory cell Cs are the same. The resistance of reference magnetic memory cell Cr can be switched by providing writing currents to word line W1 and reference bit line Br, or providing strong writing current to reference bit line Br only.
However, as array size increases, it is difficult to ensure that each memory cell and reference memory cell have the same magnetic characteristics due to manufacturing process limitations, such as magnetic resistance ratio, coercivity and interlayer coupling. Most important is uniform distribution of coercivity, which affects the working range of writing currents. Sometimes, higher writing current is required as the coercivity distribution is not uniform. However, the increased writing current can switch resistance of the reference magnetic memory cell, disturbing reading results.